Audio Source Moving @ Supersonic Speed

  • Thread starter heavyrocks
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  • #1
Let's say I have an indestructible speaker and I launch it out of a cannon at Mach 1.5 while it's playing music. (To make things simple, it also has an iPod and battery attached to it.) If the speaker passes a stationary observer, will they hear each cycle of the sound wave from the speaker in reverse order after the speaker passes them?

I remember reading a paper about this, but I can't find it anywhere now and I'm starting to think maybe I imagined it.
 

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  • #2
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I remember reading a paper about this, but I can't find it anywhere now and I'm starting to think maybe I imagined it.
xkcd: What if?

In theory, it would be possible to hear it backwards, but in a realistic setup this is really hard.
 
  • #3
russ_watters
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I can't imagine why they would hear music play backwards. While it is behind its shockwave you hear nothing, while on the same side, you hear Doppler shifted music.

If the music starts before being shot by the cannon, you hear normal music, then a sonic boom, then the more normal music combined with skipped-ahead, Doppler shifted music.
 
  • #4
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See the explanation I linked. With the example of Mach 2: 1/2 second after the radio passes you, you hear sounds emitted 1/2s ago (emitted in a distance of 1 "sound-second"). 1 second after the radio passes you, you hear sounds emitted 1s ago (in a distance of 2 "sound-seconds"), and so on.

If the radio moves at Mach 1, you hear "everything at the same time", and below Mach 1 you hear it in the right order.
 
  • #5
MFB, you're right on the money, that's where I saw it. Thanks a googol.
 
  • #6
russ_watters
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I guess the part I'm not getting is how the sound can "chase" the speaker. Any sound moving forward gets collapsed into the shock wave, doesn't it?
 
  • #7
At exactly mach 1, there is just a pressure front, but once you go faster than the sound waves themselves, all the wave fronts except the one you're currently emitting are behind you.
 
  • #8
russ_watters
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Hmm. Looking at the diagram on the wiki page for sonic boom, the shock wave forms a tangent to the expanding circles of sound and due to the angle, there is a small portion of the forward facing hemisphere not included in the shock wave. So it would indeed enable hearing some* of the previous emitted sounds backwards in a short time.

*Based on the angle emitted, you only get sounds angled slightly forward of straight down.
 
  • #9
russ_watters
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At exactly mach 1, there is just a pressure front, but once you go faster than the sound waves themselves, all the wave fronts except the one you're currently emitting are behind you.
Behind the flying speaker, yes: and almost half is combined into the shock wave.

(Edit). I'm still thinking this through/ trying to diagram it; it is hard to take the tangent of a point...
 
Last edited:
  • #10
Assuming the shock wave doesn't ruin your ears/science-grade microphone.
 
  • #11
At the apex of the cone the math gets hairy, frequency becomes arbitrarily large.
 

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